Class 5 semaphorins mediate synapse elimination and activity-dependent synaptic plasticity in hippocampal neurons.

Although autism spectrum disorders (ASDs) have long been known to have a strong heritability, the genetic basis of these disorders has remained largely elusive. Hundreds of genes have been linked to ASDs, but most of them only contribute a small increase in risk. In 2009, a genome-wide association study identified Semaphorin 5A (SEMA5A) as a novel autism susceptibility gene. Sema5A is a member of the Semaphorin family consisting of secreted and membrane-associated proteins characterized by the Sema domain. Although initially identified as axon guidance cues, Semaphorins have been found to play numerous key roles in the development and function of the nervous system. Here, I provide evidence that Sema5A, along with Sema5B, regulates dendritic morphology and excitatory synaptic elimination in hippocampal neurons. The overexpression of Sema5A/Sema5B negatively impacted dendrite complexity and reduced excitatory synapse density without affecting inhibitory synapses, in contrast the knockdown of Sema5A/Sema5B increased excitatory synapse density. I also investigated the relationship between Sema5A/Sema5B and activity-dependent plasticity including long-term potentiation (LTP) and long-term depression (LTD), which are cellular models of learning and memory. It was demonstrated that the overexpression of Sema5A/Sema5B attenuated the LTP-mediated increase of synapse density, whereas the knockdown of Sema5A/Sema5B blocked the LTD-mediated decrease of synapse density. Furthermore, soluble Sema5A treatment altered the surface expression of the AMPA receptor subunit GluA1 with total level of GluA1 unchanged. Finally, I examined the signaling mechanisms of Sema5A-mediated synapse elimination and plasticity. I found that in vitro Sema5A signalled through two members (Plexin A1 and Plexin A2) of the Plexin family, which are known as the neuronal receptors for the Semaphorin family. Moreover, TAG-1, a cell adhesion molecule also known as Contactin-2, was necessary for the function of Sema5A and Sema5B. Lastly I found that ALLN, an inhibitor of protease calpain, significantly rescued Sema5A-mediated synapse elimination, suggesting that calpain was downstream of Sema5A signaling in hippocampal neurons. Thus, my data revealed a new role for class 5 Semaphorins in synapse density and plasticity, and may therefore provide insights into the critical roles of Sema5A in the general mechanisms of circuit formation and the specific etiology of ASDs.Medicine, Faculty ofGraduat